Lyme disease is a multi-faceted illness, initiated upon infection with the spirochete borrelia burgdorferi (Bb). One manifestation of the disease is arthritis, which can result in treatment-resistant chronic arthritis in a small subset of exposed individuals. The prevalence of HLA.DR4 related alleles in these patients is an indication of an autoimmune process. They have explored the role of T helper cells in the development of Lyme arthritis, and the investigators hypothesize that patients with these particular HLA.DR alleles are at risk of developing chronic disease as a result of a vigorous inflammatory Th response to Bb. The interferon gamma production by these activated Thl cells, as well as the presence of Bb, itself leads to upregulation of MHC class II, LFA-1 and ICAM-1 expression predisposing for an autoimmune process by molecular mimicry. Based upon homology to a highly antigenic epitope of outer surface protein A (Osp A) of Bb, the applicants have identified a candidate autoantigen, hLFA-1. The aim of this application is to verify this hypothesis by creating in vitro and in vivo test systems. I. The functional similarity between the homologous epitopes and Osp A Bb and human LFA-1, but not mouse LFA-1, will be analyzed by three in vitro approaches: (a) T cell hybridomas will be generated from DR4 transgenic mice after immunization with Osp A, and the resulting clones will be tested for reactivity to human LFA-1. If double-reactive hybridomas are identified, the antigenic epitopes will be mapped; (b) synovial fluid T cells from chronic Lyme patients will be transformed and tested at the single-cell level for reactivity to Osp A and human LFA-1; (c) Osp A will be tested for binding to ICAM-1. Competition assays will be performed between Osp A and LFA-1. II. Murine models for treatment-resistant chronic Lyme arthritis will be generated by two approaches; (a) LFA-1/DR4 double transgenic mice on an MHC class II-/-background will be created and tested for the development of chronic Lyme arthritis after exposure to Bb. This is based on the observation that mouse LFA-1 does not express the Osp A cross-reactive epitope; (b) Hu-SCID-beige mice will be generated with PBMCs from patients with chronic Lyme arthritis. These mice will contain the non-immune lymphocytes of the susceptible genotype, allowing a detailed analysis of the acquisition of chronic Lyme arthritis after injection with Bb. These investigations, they propose, should provide a vigorous test of their working hypothesis, and will lead to new insights into the mechanism of treatment-resistant chronic Lyme arthritis.